A nonaqueous electrolyte secondary battery using lithium ions has been recently developed. Such a nonaqueous electrolyte secondary battery has a high energy density and is expected to be used as a power source for hybrid vehicles, electric cars, an uninterruptible power supply for base stations for portable telephone, and the like. For this, the nonaqueous electrolyte secondary battery is desired to have other performances such as rapid charge/discharge performances and long-term reliability. For example, a nonaqueous electrolyte battery enabling rapid charge/discharge not only remarkably shortens the charging time but also makes it possible to improve performances of the motive force of a hybrid vehicle and the like and to efficiently recover the regenerative energy of them.
In order to enable rapid charge/discharge, it is necessary that electrons and lithium ions can migrate rapidly between the positive electrode and the negative electrode. When a battery using a carbon based material in the negative electrode repeats rapid charge/discharge, dendrite precipitation of metal lithium is occurred on the electrode, raising the fear as to heat generation and fires caused by internal short circuits.
In light of this, a battery using a metal composite oxide in place of a carbonaceous material in the negative electrode has been developed. Particularly, in a battery using a titanium based oxide as the negative electrode active material, rapid charge/discharge can be performed stably. Such a battery also has a longer life than those using a carbonaceous material.
However, titanium based oxide has a higher potential than carbonaceous material relative to metal lithium. Further, titanium based oxide has a lower capacity per mass. Thus a battery using titanium based oxide as the negative electrode active material has a problem that the energy density is lower.
The potential of the electrode using titanium based oxide is about 1.5 V based on metal lithium and is nobler than that of the electrode using carbonaceous material. The potential of titanium based oxide is due to the redox reaction between Ti3+ and Ti4+ when lithium is electrochemically inserted and released and is therefore limited electrochemically. Further, there is the fact that the inserted and released of lithium ions by rapid charge/discharge is possible at an electrode potential as high as about 1.5 V. It is therefore substantially difficult to drop the potential of the electrode to improve energy density.
As to the capacity of the battery per unit mass, the theoretical capacity of titanium dioxide having an anatase structure is about 165 mAh/g and the theoretical capacity of a lithium-titanium composite oxide such as Li4Ti5O12 is also about 170 mAh/g. On the other hand, the theoretical capacity of a general graphite type electrode material is 385 mAh/g or more. Therefore, the capacity density of titanium based oxide is significantly lower than that of the carbon type material. This is due to a reduction in substantial capacity because there are only a small number of equivalent lithium-absorbing sites in the crystal structure and lithium tends to be stabilized in the structure.